Reactive gases are extremely useful in dry chemistry operations. For example, reactive oxygen can be used to strip photoresist from a semiconductor, and reactive nitrogen can be mixed with silicon compounds to deposit silicon nitride films on substrates.
In these dry chemistry operations, it is desirable to achieve a uniform process by employing a reactive gas which is as uniform as possible. The common method of producing reactive gases is by microwave excitation. Microwave reactive gas production devices are typically tuned waveguides with an applicator at one end. The applicator is simply a shorted waveguide with a gas flow tube running through it. The gas to be excited into a reactive state is pumped through the tube at pressures of approximately 10 Torr, and the microwave field in the waveguide is coupled to the gas to produce a plasma which excites the gas molecules to create the high energy reactive state.
There are several problems with this approach to reactive gas production. First, the microwave source must be tuned to match the impedance of the load. Since the load impedance changes with changes in gas pressure and composition, impedance matching must be performed before each production run. Also, since the impedance of the gas changes as it is excited, during the course of a production run the device must be tuned. Impedance matching is typically accomplished by measuring the forward and reflected power with a separate directional coupler disposed adjacent to the microwave source and adjusting tuning stubs in a separate tuning module disposed between the directional coupler and the waveguide to minimize the reflected power. Thus, the physical size of the separate directional coupler and tuning module make the device impractical for operations in which compact reactive gas generation equipment is required. The tuning may be done manually or with relatively complex automatic tuning equipment, but in either case is costly in production downtime or capital equipment costs.
Even more basic than these physical size and tuning problems is the inherent limitation of the microwave devices. With a shorted waveguide applicator, the gas pressure range over which a reactive gas discharge can be initiated is limited. This is due to the fact that the field in the applicator is simply whatever field is propagated in the waveguide, which severely limits the range of acceptable load impedances. In addition, the nonuniformity of the field in the applicator produces a nonuniformly energized reactive gas, which may contribute to nonuniform downstream processing. This is unacceptable for processing of integrated circuits and other structures in which the uniformity of the gas is critical because of the narrow processing tolerances.
Because of these problems, microwave devices have not been able to fill the need for a reactive gas generator which is compact, simple to use, and effective with a variety of gases at a wide range of pressures and flow rates.